The invention relates in particular to a system in which the method of distance measurement or positioning employed comprises:
obtaining successively a series of primary phase measurements, each primary phase measurement being obtained by: PA1 combining the primary phase measurements of each of a plurality of selected pairs of the primary phase measurements in such a manner as to produce a secondary phase measurement in which any instrumental phase error incorporated in the respective primary phase measurements is substantially eliminated, each secondary phase measurement corresponding to a measuring frequency which is determined by the pattern and auxiliary pattern frequencies at which the primary phase measurements of the respective pair are obtained; PA1 the pattern and auxiliary pattern frequencies and the manner in which the primary phase measurements are combined being selected in such a manner that the distance can be derived from the secondary phase measurements unambiguously to the wavelength of the lowermost measuring frequency and to an accuracy equal to a fraction of the wavelength of the highest measuring frequency. PA1 .lambda.=the effective wavelength of the continuous wave; PA1 .alpha.=the phase measurement expressed as a fraction of a whole phase rotation; and PA1 n=the unresolved number of whole phase rotations. PA1 control means; PA1 first generating means responsive to the control means for generating a pattern signal having any selected one of a series of accurately known pattern frequencies; PA1 second generating means responsive to the control means for generating an auxiliary pattern signal having a frequency which differs from the selected pattern frequency by a predetermined off-set frequency; PA1 means for transmitting the pattern signal so as to be propagated electromagnetically; PA1 means for receiving the pattern signal after said propagation; PA1 means for mixing the pattern signal and the auxiliary pattern signal with one another before and after said propagation, thereby to provide two comparison signals each having a frequency equal to the off-set frequency; and PA1 means for carrying out a phase measuring operation on the two comparison signals to provide a primary phase measurement representative of the phase difference between the two comparison signals; PA1 the control means being operative:
generating a pattern signal having an accurately known pattern frequency; PA2 generating an auxiliary pattern signal having a frequency which differs from the pattern frequency by a predetermined off-set frequency; causing at least the pattern signal to be propagated electromagnetically; PA2 mixing the pattern signal and the auxiliary pattern signal with one another before and after said propagation, thereby to provide two comparison signals each having a frequency equal to the off-set frequency; and PA2 carrying out a phase measuring operation on the two comparison signals to provide said primary phase measurement as the phase difference between the two comparison signals; and PA2 to cause the first generating means to generate successively a plurality of pattern signals each having a selected one of said pattern frequencies, and to cause the second generating means to generate the corresponding auxiliary pattern signal; PA2 to combine the primary phase measurements of each of a plurality of selected pairs of the resulting primary phase measurements in such a manner as to produce a secondary phase measurement in which any instrumental phase error incorporated in the respective primary phase measurements is substantially eliminated, each secondary phase measurement corresponding to a measuring frequency which is determined by the pattern and auxiliary pattern frequencies at which the primary phase measurements of the respective pair are obtained; PA2 to select the pattern and auxiliary pattern frequencies and the manner in which the primary phase measurements are combined in such a manner that the distance can be derived from the secondary phase measurements unambiguously to the wavelength of the lowest measuring frequency and to an accuracy equal to a fraction of the wavelength of the highest measuring frequency; and PA2 to select the pattern and auxiliary pattern frequencies and the manner in which the primary phase measurements are combined in such a manner further that at least one of said measuring frequencies (other than the highest) is related to the next higher measuring frequency by a multiplying factor .beta. which lies within the following range: EQU 1&gt;.beta.&gt;1/8.
Said propagation usually takes place as modulation of a suitable, electromagnetically propagated carrier wave. Carrier waves of a relatively high frequency are normally used, as these permit collimation into narrow beams, and as their speed of propagation is not, to any significant extent, affected by ground conductivity.
Such a system is, for example, described in U.S. Pat. No. 2,907,999 to Wadley and is herein also referred to as a system of the Wadley type.
As disclosed in the above Wadley patent, the Wadley system can be used to measure the distance between two stations, or, in positioning a station, to determine the difference in distance of that station to two other stations, thereby locating that station on a hyperbolic locus having the two other stations as foci. Unless the context indicates otherwise, reference herein to distance measurement should be construed as including positioning of a station with respect to two or more other stations.
Although systems of the Wadley type are very advantageous as far as accuracy is concerned, they are disadvantageous as far as the speed of measurement is concerned. This is a drawback particularly in dynamic applications where a number of primary phase measurements are made not simultaneously but at different times.
In electromagnetic distance measuring systems generally, distance is essentially derived from the time it takes for a signal to be propagated electromagnetically along a propagation path corresponding in length to the distance to be measured. In continuous wave systems (such as the Wadley system), as opposed to pulse systems, the signal is a sine wave signal of accurately known frequency, and the time is, in effect, measured by determining the phase delay experienced by the signal in being propagated along the propagation path. Knowing the phase delay and the wavelength of the sine wave resulting from electromagnetic propagation of the signal, the distance can be determined.
To determine the phase delay, the phase of the signal before propagation is compared with the phase of the signal after propagation. This results in a phase measurement which represents only the fractional part of the phase delay. In other words, although a phase measurement at a particular frequency can be used to resolve the phase delay to a fraction of a whole phase rotation, it cannot be used to resolve the number (if any) of whole phase rotations. Mathematically, the distance to be measured (D) can be expressed as follows: EQU D=(n+.alpha.).lambda.
Where
The effective wavelength is that length by which the distance D must be increased to increase the phase delay by one whole phase rotation.
n is also referred to as the ambiguity factor, and represents the ambiguities in D resulting from the unknown number of whole phase rotations.
The accuracy to which the distance D is required to be measured in practice is typically in excess of one part in 10.sup.5. However, there are limitations on the accuracy to which the phase measurement .alpha. can be made. In most situations, these limitations make it impossible to determine D to the requisite accuracy, with a phase measurement made at a frequency for which .lambda. is known to be greater than D (i.e. n=0). It is therefore usually necessary in continuous wave systems to obtain a phase measurement at each of a number of different frequencies, the highest frequency being selected to provide the requisite accuracy, and the lower frequencies being selected to resolve the ambiguities. Put in a different way, the lowest frequency is selected to provide a coarse but unambiguous measurement of the distance, and the higher frequencies are selected to upgrade the accuracy of the coarse measurement without introducing ambiguities.
In one particular form of the Wadley system (which is here described for purposes of illustration only), the highest pattern frequency is referred to as a reference pattern frequency or simply the reference frequency fPr, and the other pattern frequencies fP1, fP2, etc are selected in such a manner that they are related to the reference frequency as follows: EQU fM1=fPr-fP1 EQU fM2=fPr-fP2, etc
The reference frequency represents the highest measuring frequency, and fM1, fM2, etc represent the lower measuring frequencies.
The secondary phase measurement .alpha.Mr corresponding to the highest measuring frequency fPr is derived from two primary phase measurements .alpha.(plus) and .alpha.(minus), the first being obtained with the corresponding auxiliary pattern signal having a frequency higher than that of the reference frequency, and the second being obtained with the auxiliary pattern signal having a frequency lower than that of the reference frequency. The phase measurement .alpha.Mr is derived as follows: EQU .alpha.Mr=1/2(.alpha.(plus)-.alpha.(minus))
The secondary phase measurements .alpha.M1, .alpha.M2, etc corresponding to the lower measuring frequencies fM1, fM2, etc are derived from the primary phase measurements .alpha.Pr, .alpha.P1, .alpha.P2, etc obtained at the various pattern frequencies, as follows: EQU .alpha.M1=.alpha.Pr-.alpha.P1 EQU .alpha.M2=.alpha.Pr-.alpha.P2, etc
More generally, where the auxiliary pattern frequency for the one primary phase measurement is higher than the pattern frequency and for the other phase measurement lower than the pattern frequency, then the corresponding measuring frequency is equal to twice the pattern frequency where the pattern frequency remains the same, or equal to the sum of the two pattern frequencies where they differ. Where the auxiliary pattern frequencies for the two primary phase measurements are both higher than or both lower than the corresponding pattern frequencies, then the corresponding measuring frequency is equal to the difference between the two pattern frequencies.
As is known in the art, and as will be described hereinafter, derivation of the secondary phase measurements in this manner leads to the substantial elimination of instrumental phase errors.
By suitable selection of the various pattern frequencies it is then possible, from the secondary phase measurements .alpha.Mr, .alpha.M1, .alpha.M2 etc., to determine the distance D unambiguously, to the requisite accuracy.
Mathematically, the relationship between the various measuring frequencies can be expressed as follows: EQU fM1=.beta.fPr EQU fM2=.beta.fM1 EQU fM3=.beta.fM2, etc.
where .beta. is a multiplying factor which need not necessarily be the same for each measuring frequency, but which is such that fM1 is lower than fPr, fM2 lower than fM1, and so on.
It will be appreciated that the various phase measurements .alpha.Pr, .alpha.P1, .alpha.P2, etc can be obtained in any desired sequence.
The value of .beta. should not be smaller than twice the worst case error in .alpha.(expressed as a fraction of .alpha.). If it is, then there is the risk that a breakdown in ambiguity resolution may occur.
The accuracy to which .alpha. can be determined is limited by inaccuracies introduced by transmission noise and inaccuracies introduced by multi-path reflections. Multi-path reflections are unwanted reflections of the carrier wave from objects (usually the ground) alongside the propagation path. The inaccuracies introduced by transmission noise, expressed as a fraction of .alpha., do not vary significantly with the wavelength .lambda. or with the carrier frequency, although they do vary with distance and propagation conditions (e.g. weather conditions). The inaccuracies introduced by multi-path reflections, being dependent on actual measuring conditions in the field, are basically unpredictable, and can only be assessed on a statistical basis from experience. Even in a simple case where only one multi-path reflection is present, the effect depends on many factors such as the shape, size and reflection coefficient of the reflecting surface, relative angle of polarisation of the signal relative to the reflecting surface, the excess pathlength of the reflection path, the pattern frequencies, the .beta. factor, and the carrier wave frequency. However, it has been found that where secondary phase measurements, each derived from the difference between two primary phase measurements obtained with the auxiliary pattern frequency in both cases being higher than or in both cases being lower than the corresponding pattern frequency, are used to resolve ambiguities as described above, the effect on the secondary phase measurements of multipath errors that have equal chances of occurring for different .beta. factors, does increase with .beta., but less than proportionally. It has also been found that, with .beta. values of 0.1 as used in the majority of Wadley type surveying instruments, failure, as a result of multi-path errors, to resolve ambiguities, is rare. The effect of inaccuracies introduced by transmission noise can be reduced by averaging the phase measurement over a length of time, herein referred to as the averaging time. Such averaging does not, however, reduce the effect of inaccuracies introduced by multi-path reflections.
In previously proposed Wadley systems the .beta. factor was chosen to be as small as possible with what were considered resonable averaging times. Typically, .beta. factors of 0.1 for systems based on a microwave carrier were used. This required an averaging time in the order of 0.5 second per phase measurement in order to reduce the worst case error in .alpha. to below the required 0.05. Although such an averaging time is acceptable for static measurements, i.e. where the distance to be measured does not change significantly during the time interval between successive phase measurements (from the start of one phase measurement to the start of the next), problems arise when there is a substantial change in the distance during that time interval.
Where the rate of change of distance is constant, the problem can, to a certain extent, be overcome by making a phase measurement at the reference frequency fPr before and after a phase measurement at a pattern frequency fP1, fP2, etc, and by making use in the subsequent derivation of the secondary phase measurement, of the average of the two phase measurements at the reference frequency. This technique is also referred to as straddle measuring or sandwich measuring. This method of eliminating the effect of a changing distance rapidly fails if the rate of change itself changes, e.g. if the distance to an overflying aircraft is being measured. Even where the rate of change of distance does remain constant, there is a limit beyond which the technique of straddle measuring leads to ambiguous results. This limit is exceeded when, during the time interval between two successive phase measurements at the reference frequency, the distance changes by more than 1/2.lambda. of the reference pattern signal. In a typical system designed for static conditions, where the wavelength of the reference frequency is 10 m, and the averaging time which is required to reduce the worst case error in .alpha. to less than 0.05 is 1 second, ambiguity resolution of the system would break down when the distance to be measured changes at a rate of more than 0.5 m/sec or about 1.8 m/h. If the straddle or sandwiching technique were used, this would increase to 2.5 m/sec or 9 km/h. This is assuming that the rate of change is constant. Quite clearly such a system would not be suitable for accurately measuring the distance to even a slow-moving station such as a ship--let alone a fast-moving station such as an aircraft.
In previously proposed Wadley systems, the problem has been dealt with by reducing the averaging times as far as possible without reducing the maximum range specifications unduly, and mainly by increasing the wavelength of the reference frequency--typically to 100 m or more. The ratio of speed to wavelength decreases as the wavelength is increased, so that the effect on .alpha. of changes in distance with respect to time decreases in proportion to the increase in wavelength. The accompanying loss in accuracy was accepted as inevitable.
It is an object of the present invention to provide a continuous wave electromagnetic distance measuring system of the Wadley kind, which is better able to deal with the measurement of a changing distance. It is also an object of the invention to provide such a system which is able to provide an increased measuring speed and/or resolution of distance and which is less sensitive to errors introduced by multi-path reflections.